


I Burned the Bridge Across the Rubicon

by ohjustdisarmalready



Series: The Road Goes On [3]
Category: Undertale (Video Game)
Genre: Alternate Universe - Underfell (Undertale), Brotherly Angst, Brotherly Love, Dramatic Irony, Family Issues, Gen, Underfell Papyrus (Undertale), Underfell Sans (Undertale), but still sweet, chara is there for a hot second but no one can see them so..., do u feel them, do u feel ur sins yet red, uf!Papyrus is a cinnamon roll that's been burned a few too many times
Language: English
Status: Completed
Published: 2020-06-17
Updated: 2020-06-17
Packaged: 2021-03-03 23:00:55
Rating: Teen And Up Audiences
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,444
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/24763498
Author URL: https://archiveofourown.org/users/ohjustdisarmalready/pseuds/ohjustdisarmalready
Summary: Chara's host is gone.Sans's deed is done.Papyrus's brother comes home alone.
Relationships: Frisk & Papyrus (Undertale), Papyrus & Sans (Undertale)
Series: The Road Goes On [3]
Series URL: https://archiveofourown.org/series/1790866
Comments: 15
Kudos: 37





	I Burned the Bridge Across the Rubicon

**Author's Note:**

  * For [Elara_Moon](https://archiveofourown.org/users/Elara_Moon/gifts).



> Aha! I said I'd do requests for Whither Then, and I have finished my first one! Is it because it was halfway written already? Who's to say?
> 
> For the uninitiated, this is in a universe where Underfell!Sans pushed Frisk into the next universe over, Underswap. This is the immediate aftermath of that decision.
> 
> The first half of this is probably **not canon** for the story--I want to refine Red's thought patterns a little more before I commit to his POV in such a pivotal scene. This is close, but not precisely what I'll end up with, I think. The second half, however, is going to stay. This is what happens when he goes home afterwards.

Chara’s host falls away from them—how? What did he do? What did he do to them?

 _I told you to run_ , they try to think at Frisk, but no one is there to hear it.

What has Sans done? What could he have possibly done, to make it so that Frisk is just _gone_? There was no RESET, no…nothing.

Just this hole in the world and this empty lab.

What happened? How did this happen?

Chara is alone.

* * *

…Emotionally, they’re alone. Physically, someone else is in the lab, of course. Someone had to work hard to open this portal, after all. Long, hard nights of complicated math. Coffee grounds straight from the bag. More math. Hit it with a wrench a few times.

And now, his efforts have paid off.

Sans huffs out a surprised breath, almost like a laugh, hardly daring to hope. He looks through the lab, but as far as he can tell, it’s really empty. No one in here but him. No…time travel, no coming back—he really took them by surprise, and now they’re really gone, right?

It worked…? It really…?

The thought is nearly incomprehensible. Sans just hadn’t realized how much he was expecting something to come up, or for it to fail somehow.

He’d run the calculations, sure, but some part of him had been convinced that it couldn’t possibly—he’s never had a breakthrough with the machine before, not ever.

At least, not one usable for a monster like himself, unless he wants to shatter into pieces—at best. For a human, on the other hand…DETERMINATION is a hell of a thing. Opens up so many possibilities…

He’s pretty sure they shouldn’t be able to, but just in case the kid can somehow come back from that, Sans sets to work shutting the portal as quickly as he can. He’d calibrated it to open automatically with his shortcut, but closing it…he flips two switches on the side of the machine itself, and presses three buttons on a nearby keyboard in his mess of a lab.

Then, with the last of his magic, he throws a bone attack through the power main.

Yep. Looks good. The portal zips itself shut with a _whoosh_ of interdimensional gravity, and soon, there might as well have never been a seventh human.

Sans stares at the space where the portal was for a moment. An odd feeling seizes him by the ribs. He really did it…he really…

He feels light in a way he doesn’t think he ever has. It’s unreal—he feels unreal.

He’s so relieved. Elated, even. Sans has never been elated in his life, but he _did it_ and it’s _done_. He was dead certain that the kid would come up with something, avoid it somehow, already know what was coming…but they were taken completely by surprise, almost willfully so. They just let their guard right down, exactly how he and Papyrus are always telling them not to. Didn’t even fight him until the last possible second, and by then it was too late.

A feeling tries to pierce its way into Sans’s heart—something like grief, something like fury—before he waves it off with an irritable grumble. This is good. He did it. The kid is gone, now, safely tucked away forever. He’s not gonna ever see them out of stripes or kick their knees out from under them when they get taller than him or celebrate another birthday with them or even learn their name—

Something tugs a little at his SOUL.

Okay, maybe he was hoping just a tiny bit that they could be content living in Snowdin with Sans and his brother forever.

Or, not forever, obviously. That’s just impractical. But they could have stayed for a little while longer. Just until they were old enough to understand, just broken down enough to _ask_ Sans to save them instead of kicking and screaming about it. Maybe a decade or so. Long enough to hit some milestones, maybe—obviously the first kill is a non-starter, but—

Hell, what is he thinking? Stupid fantasies don’t matter. What matters is reality, and family, and what Sans has finally, finally succeeded in. It’s over, now. He did it. There’s no turning back.

Besides, sentencing the kid to grow up in a hellhole like this would pretty much make him a bad person. Sans isn’t exactly an altruist, but he knows when the greater good has to win out. Sans did the _right thing_. The kid is—was—too young to understand, but figuring out how to make that portal was the most ‘right’ thing Sans has done in a long, long time.

He did for them what he failed to do for Papyrus; what Papyrus has always resented him for not doing. He protected them from the evil in the world. He’s basically provided a happy upbringing for them, because that idiot creampuff version of him would rather implode than hurt a kid. In one aspect of his life, with one member of his makeshift little family, Sans _hasn’t fucked up_.

…besides. He gave them an out. He would have let them stay safe in Snowdin, if they were willing to stay. It’s not his fault they didn’t take him up on it.

What he’s feeling isn’t grief. He’s not…feeling anything. He’s relieved. That’s all it is. He should be happy, shouldn’t he? He worked so hard for this. He worked—so, so hard for this. And it worked. This was the kindest way. This was the _right_ way. This was the only way. He couldn’t have them coming back.

_They trusted you. They wanted so badly to trust you._

Probably what he deserves. There’s a reason he’s stuck here with the rest of these bastards, and it’s not just to be a live-in jail warden.

Welp. The human’s…heh. They’re in a better place now. And Sans is…here, still. For as long as he lives. Acting out his judgement against murderers and monsters of every sort.

It’s a bad idea to wish he went with them, right? That’s probably no good. Pap’d be pissed.

Eh. He’s gonna be pissed anyway, if Sans doesn’t get up there for supper soon. Time to choke on some lasagna abomination more bitter than his sins.

…or, he will, soon. He’s just gonna…stay in the lab for a minute, first. Just. Watch where the portal closed.

Just in case, somehow, they come back home.

* * *

Papyrus notices immediately when his brother walks in the door. Sans may not have as much EXP as most Boss Monsters, but anyone with that kind of LV walking into a room is going to draw attention—especially the attention of someone like Papyrus, who has use for his life and will make moves to defend it.

Even if he doesn’t have Sans’s ability to perceive someone’s LOVE precisely, there’s a sort of…urgency, perhaps, that one feels when someone dangerous enters the room—that is, when anyone enters the room. Normally, with Sans, it’s a mere tingle of awareness; Sans is mostly made of wasted power and idle threats.

What’s strange now is that his brother’s LV is heavier than usual…and he doesn’t have the human with him. He said he was going to go meet them, didn’t he?

“Sans,” he barks. “What has happened?”

Sans jumps—guilty—and attempts to shrink into the doorway to the same time that he closes the door. Not only is his grin even emptier than usual, but he’s actually using a door like a regular person? Something is certainly wrong.

“eh. human’s prob’ly about where i left ‘em. give or take. they were bein’ a stubborn little…” He stops as Papyrus clears his throat. He has made it clear that there will be no insulting other members of his household under his roof! “uh. kid. stubborn…kid.”

Hm. Odd that they have not returned, but not necessarily alarming, if Sans has been by to check on them. Slob he may be, but Papyrus’s brother is diligent enough when it comes the human. And the human has been known to insist on behaviors that don’t make sense; so they do come home at odd hours from time to time. They still refuse to use the many bone attacks Papyrus has furnished them with, no matter how many FIGHTS they encounter.

“And your LV?” Papyrus prompts, since Sans is clearly done discussing the matter of the human. Papyrus is unconcerned; they will return in due time, safe and sound. They always do.

He will call them once he and his brother have eaten, just in case. They will be expecting it, unless they have gotten caught up in something and have forgotten the time. Papyrus calls them every night after supper if they are not home by then—it wouldn’t do to have people thinking they are unwatched.

Sans is avoiding his gaze.

“Sans. Your LV. Do not claim that you thought I wouldn’t notice.” Papyrus crosses his arms with an imposing scowl. It’s very frightening—he practices it in the mirror each morning.

Sans slouches over to the couch and collapses on it with a frown of his own. “ran into a kid.”

Oh.

This must have been after checking on the human, then—or, perhaps the human witnessed the encounter, and was frightened? No, they aren’t squeamish about the fact that Sans and Papyrus have killed before and will likely do so again.

Then again, encounters with children are especially…difficult, psychologically.

The killing of children is not specifically addressed by law in the Underground—children are an easy source of LOVE, for some. Those monsters without the power or cunning to protect themselves as children would not survive adulthood, and so there is no reason to spare them in their youth. Some families quietly guard their children at home, or protect them in other manners, but on the whole, children are only as safe as they can make themselves.

That being said, Sans and Papyrus have not killed children since they were in stripes themselves. Not…when they could help it.

LOVE is necessary to survive. Children are born without it. Therefore, they need to gather it somehow—if not by killing other children, then by stalking and ambushing an older monster. Sometimes, a particularly cocky youngling will announce their desire to kill Papyrus for fame and LOVE. If he can discreetly spare them, he does, but some are persistent.

When it comes down to it, Papyrus needs his reputation for heartlessness—to protect himself and his brother and his human—more than he needs an easy conscience. And leaving a child alive who has proved that they can’t be dissuaded from attempting to kill him will just lead to that child getting lucky, one day. No one wants to be dusted by a freak accident and a child.

Only twice has Papyrus been unable to avoid killing a child as an adult himself—once in Snowdin Square, too public to offer MERCY more than once; and once in desperate circumstances. Both made for a small bump in his EXP—not difficult kills—but a large leap in LOVE.

There are few acts more violent than the killing of a child.

Papyrus is tempted to call his brother an idiot for not being able to avoid this—Sans is smarter than that, has no need to maintain a reputation when Papyrus’s keeps them protected enough, and surely no child could really force Sans to act violently if he truly wanted to avoid it, surely Sans was just careless and needs to put more effort into his _everything_ —

But. Papyrus is a good role model. It wouldn’t do for the human to come home to him yelling at Sans for a kill that, in truth, must have been unavoidable; not when they’re so hesitant to kill already.

And it _must_ have been unavoidable. Papyrus knows his brother—Sans can rarely drudge up the intent to harm anyone with low LV. He puts on a good show, sure, and he may believe it himself; but if he thinks Papyrus buys into his bullshit then he’d better think again.

Sans needs to feel justified in his kills, or he’ll spend days listlessly staring into nothing and muttering darkly. It’s a hypocritical crutch, one that Papyrus hates more than any of his brother’s many flaws—Sans’s selfish delusion that he can be any better than the rest of them, that his life is bought any cheaper just because the dust isn’t on his own hands. Frankly, at one point, it became easier for Papyrus to just do the dirty work and let his brother live in a fantasy of justice.

But he’s trying not to think that way anymore. He’s trying to resent his brother less for his failures and delusions. They’re both adults, now, and Papyrus is trying not to hold it against him that Sans somehow got to keep his ideals while Papyrus did not.

Sans can be judged for his cowardice, his hypocrisy, his willful ignorance to the things Papyrus has been forced to do to keep the two of them alive because Sans just won’t stoop so low as to survive; but Papyrus has been wondering, lately. If he teaches the human to judge Sans and find them wanting, then what will they one day think of Papyrus, whose hands are thick with dust and whose LOVE is greater than anyone in Snowdin’s? What will they do when they realize how far Papyrus has gone to protect his family?

Papyrus is honorable and self-controlled. He is not prone to violent rages. And yet, before the human came, Sans had neatly slid him from ‘us’ into ‘them.’ With only the best intentions, keeping killing to a minimum just the way Sans had always said was _right_ , Papyrus has been judged and condemned.

And now…now that Sans is finally softening a touch; willing to concede to being cordial with Papyrus, if only for the human’s sake…now that they are almost, nearly brothers again; clicking undeniably like two people who have spent their lives together must; Papyrus can’t stand to let his family fall apart again. He will not be the one to drive them all apart.

He has to remember that children copy their elders; and the behavior he displays is the behavior he will one day receive. God knows Papyrus had adored Sans right up until Sans had stopped adoring him back.

Papyrus is a good role model, and he will not have his human walk in on him shouting at his brother for a necessary act of violence.

…Papyrus is not certain what to say, if it is not to berate his brother for getting into ~~worrying~~ aggravating situations.

He’s been silent for too long. Sans is on the defensive.

“look, they didn’t give me much of a choice, alright? i told ‘em to go home, told ‘em they weren’t gonna like what happens if they don’t fucking quit it, and they were still so damned—” Something raw is in Sans’s eyes, both visible for once, and his phalanges dig into the holes in the upholstery. He keeps cutting glances at the small folding chair they’ve dragged in next to the couch. Papyrus doesn’t berate him for swearing. “i, i—i did the right thing. you have no fucking right to judge me.”

 _And no desire to. Unlike some people, I do not search for excuses to_ —the retort is already building in Papyrus’s mind, but he shuts it down. No. It’s not just the two of them quietly growing to hate each other anymore. He’s going to do this the right way.

The right way, which starts with…

…what did Papyrus want to hear, desperately, all those years ago? The very first time he came home, rattling nearly out of his stripes, and immediately sought out his brother? The night Sans had taken one look at him, _through_ him, and turned away with some bullshit excuse about going to the lab? What had he needed, the night Sans looked at him and stopped seeing his brother?

Papyrus knows what to say, he’s pretty sure.

He sits on the couch next to Sans, looking straight ahead.

“I know you did,” he says.

“you—what?” Sans’s snarl falls away, and the roiling magic sifting through the room falters for a moment.

“I know,” says Papyrus. “I do not know what happened…but I know that you are my brother, Sans. I know who you are. I know that you would not harm a SOUL when given any other choice.”

The words fit awkwardly between his teeth, but they’re the right ones, the thinks.

Sans has turned to face him, but Papyrus stares resolutely at the wall. He is going to do this right, and not feel overwhelmed by old hurt. He will not look at his brother’s face, right now. Perhaps one day, the two of them and the human will mend enough of their family that he can keep his temper. For now, it takes his full attention to try to do this right.

“You are still my brother,” he says. “No matter what you do, you will be my brother. Just as the human, should they gain LOVE one day, will remain our sibling.”

Refusing to look at his brother, Papyrus misses Sans’s reaction.

“You are still the same person. The things you are forced to do…they will change you, and they will hurt. But they will not make you less than you are. And they will not change my regard for you.”

There. That is it, he is fairly certain—that is everything he’d wanted to hear, when he’d still gone to Sans for shelter. That is all he can think to say. He has already swallowed his pride enough for one evening.

Sans remains silent and still, and Papyrus risks a glance at him from the corner of his eye socket. Even with his lack of eye lights, Sans always seems to know when he’s peeking.

The look on Sans’s face is…indescribable.

“…brother?” Papyrus asks, wary. Has he finally broken Sans? Surely it cannot be _that_ unexpected for Papyrus to have a moment of kindness.

Surely, their brotherhood hasn’t fractured that far.

“i—” Sans starts, still staring at Papyrus with that complicated expression. “i—lab. i’ll be back. seeya.”

Even saying that, he doesn’t move. Papyrus doesn’t wait for him to get his wits together.

“Absolutely not,” he says. “Take a shower. Scrub your filthy bones; it’s been weeks. Get some of that disgusting muck you call food. I will call the human and tell them to come home. Meet us here in thirty minutes—if I find you lying on your floor and feeling like garbage, you _will_ regret it.”

A night in surrounded by family is called for.

“wait,” Sans blurts. Papyrus pauses halfway through dialing.

“the human is—busy. they’re—uh—just—just leave it alone for tonight, ok? don’t—don’t worry about it. just don’t call them,” he says. “just, leave it alone for now, papyrus. please.”

Most likely, the human witnessed Sans’s act of violence. Perhaps that is why they are late—no matter. Papyrus’s youngest sibling is known for their MERCY; they will not resent Sans for a necessary act. More likely than not Sans has decided how they feel without consulting them and accordingly run away, as he does from everything that makes him feel less than righteous.

Very well. Papyrus is certain that they’ll be alright—even at his most defensive, surely Sans would not have abandoned them in a dangerous situation. They are safe somewhere tonight. Sans and Papyrus have acquired a very DETERMINED human.

Papyrus will still call them when Sans is in the shower, to ensure that they are well and not terribly frightened, but he can certainly understand not wanting to be confronted with the consequences of one’s actions so soon after taking them. Papyrus does not go out of his way to speak with witnesses of his own most terrible actions; and on the rare occasion that Sans is one of those witnesses, he has rarely been able to look his brother in the eye socket.

Papyrus never feels right when he can’t be sure whether his littlest sibling is safe and sound, but Sans’s word is the next best thing.

Papyrus puts his phone away. Seems it will be just the two of them tonight.

“Go and get clean, brother,” he says. “I will be here.”

**Author's Note:**

> In my dreams, there's a companion piece to this that comes when Papyrus puts together, 'hey, didn't the human happen to disappear, never to be seen again, the same night as Sans felt super guilty about killing a kid...?' and details how Sans decided to contact his counterpart in Underswap. I have it abstractly spinning around in my mind, but it hasn't been defined yet. We'll learn a little more about how that goes down in Whither Then, even if I don't make a oneshot for it.
> 
> Papyrus in this context was pretty challenging but super interesting to write. He's a really complicated character...his part of the story is easily overlooked, because he hasn't shown up in the actual narrative except for in flashbacks and in his influence, but he's one of the characters who suffers the most for Sans's decision. In a way, he lost both of his siblings at once. When he actually shows up in Whither Then, it's going to be interesting :D
> 
> Please remember to comment!


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